This invention deals generally with vapor coating and more specifically with an apparatus for producing vapor and varying the quantity of vapor produced.
Molecular beam epitaxy is a sophisticated ultra high vacuum evaporation process used to create thin epitaxial layers of materials. Much of the use of molecular beam epitaxy is in the growth of semiconductors such as GaAs and AlGaAs. Molecular beam epitaxy is favored for the growth of highly complex, multilayer structures where control over layer thickness and composition is essential. Typically, layer thickness must be controlled to 3% or better and composition must be controlled to within about 5%. Structures having more that 100 individual layers, such as vertical cavity surface emitting lasers, are now routinely grown in many research laboratories.
The success of the molecular beam epitaxy process rests heavily on the ability to precisely control the delivery of extremely high purity vapors of the constituent elements. This is now done in elemental source molecular beam epitaxy by the use of mechanical shutters to abruptly block and unblock the beams of condensible vapors, and through variation of constituent element vapor pressures by means of the variation and control of temperature. The source of these vapors is usually a crucible which contains the material to be evaporated and which is heated by a tubular furnace. Under ultra high vacuum conditions and at temperatures involved with the typical elements used, heat transfer between the heater, crucible, and temperature sensing element, which is usually a thermocouple, is essentially only by radiation. To provide accurate control of the material evaporating, the so-called "flux", careful design is required to keep the spatial relationship of the temperature sensor and the crucible constant, and thereby keep a constant relationship between crucible charge surface temperature and temperature at the temperature sensor. Despite this difficulty, conventional sources maintain flux within a few percent over time periods sufficient for epitaxial growth of the vast majority of structures. Longer term flux stability is not as good, and is limited by source material depletion and mechanical changes in the source operating conditions.
Although conventional sources work well when the rate of material deposition is relatively constant over time, the design of variable rate sources is a problem. Because of the need to load the crucible with sufficient material to allow for the growth of many microns of epitaxial semiconductor, molecular beam epitaxy sources typically contain many grams of gallium, aluminum, or indium. Moreover, to reduce the thermal load to the vacuum system, a large amount of thermal shielding is usually provided, and to provide the mechanical rigidity to maintain the source charge and the temperature sensor in a fixed geometry, structural bulk is also needed. The result is a vapor source with a significant thermal mass and a reduced radiative capacity. This means that its thermal response to changes of heat input is relatively slow.
Some molecular beam epitaxy processes require that the deposition rate of the vapor source be variable over an order of magnitude or more in a time span of a few seconds or less. This is achievable only with considerable difficulty using conventional vapor source designs. The thermal mass of the source makes adjustment of the vapor source's temperature at the speed required difficult or impossible. Furthermore, the reproducibility of the resulting temperatures is poor even for those temperature changes which are possible. For example, grading the emitter-base junction of a heterojunction bipolar transistor could be done in its simplest form with an aluminum flux change of about a factor of 15 over a thickness of 100-200 Angstroms, which at typical molecular beam epitaxy rates translates to 30-60 seconds. Even the most modern, high uniformity sources are not capable of controlled temperature changes at this rate.